Conventional sonar beamformers utilize a linear array of hydrophones directly connected to a delay line to view various segments of the hydrophone sensory field in an effort to detect signals approaching from various directions.
If the signal is from a target located directly broadside to the sensor array, all of the sensors receive the signal virtually simultaneously. Consequently, there will be no delay between the time one of the sensors responds to the incoming signal and any one of the other sensors respond to the same signal. At the other extreme, the end-fire position, when the target is approaching at ninety degrees to the broadside position, the sensor closest to the target will respond to the signal first. Each subsequent sensor will then respond sequentially until the last sensor, farthest from the target responds to the signal. Consequently, there will be maximum delay between the receptions of the sensors to the signal when the target is located in this end-fire position. At positions between the two extremes the relative delay increases as the target moves from the broadside, or zero-degree position to the end-fire, or ninety-degree position. In order for all of the hydrophone outputs to be processed simultaneously, it is necessary to delay the outputs from the earlier hydrophone receptions to a greater degree than those from the later hydrophone receptions.
Conventional beamforming devices employ a delay line means to receive the sensory outputs from the hydrophone sensor array. Typically, a clock circuit is utilized to vary the rate at which the outputs are transferred through the delay line in order to provide the appropriate delay.
The clock period (delay per hydrophone) is expressed as T, where T equals the sine of the angle A of the incoming signal multiplied by the spacing, H, between the hydrophones and divided by the speed of sound, c, or T=sin A (H/c). The clock rate is expressed as 1/T.
In order to view a particular segment of the hydrophone sensory field, the clock rate is adjusted to provide the optimal delay necessary to detect a signal approaching from the particular angle of attack corresponding to that segment of the hydrophone sensory field.
In order to vary the segment of the hydrophone sensory field to be viewed, the clock rate is changed to provide the optimal delay necessary to sense a signal approaching from that new segment of the hydrophone sensory field.
This device requires a substantial amount of hardware. In order to reduce the amount and cost of hardware used, a second approach utilizes a two dimensional stepped or organ-pipe charge coupled device (CCD), in which each hydrophone output is connected directly to a different step or row of the CCD. The delay of each row of the CCD increases uniformly so that all the outputs from the sensors representative of a specific signal at a predetermined angle of incidence arrive at the output of the CCD register simultaneously for processing. The clock rate that drives the CCD is adjusted, as described above, to provide the optimal delay necessary to detect a signal approaching from the particular segment of the hydrophone sensory field that is to be viewed.
If that segment of the hydrophone sensory field that is to be viewed lies in the opposite quadrant however, it would become necessary to reverse the entire interconnection between the hydrophone sensor array and the CCD. In addition, the CCD requires a special construction in a two-dimensional array to enable it to form a single beam corresponding to a single signal.